Oyumaru/Instant Mold Casting

Oyumaru/Instant Mold Casting

Posted on 29/02/2012 by sho3box

While prepping some figures for my ongoing Adeptus Mechanicus project I decided to cast up a replacement head for one of the figures.
Way back in the last century I made more silicon rubber moulds than I could count.  I also cast very many items in various resins in them, as it was a large part of my old job working in industrial prototyping.
A lot of people seem to enjoy using those industrial substances at home in their workshops, sheds and hobby rooms and good luck to those people: I hope that they enjoy it.
I on the other hand have no desire to invest time and money in such horrible, toxic and messy substances for use in my house.  Therefore any casting that I do has to be clean, easy and effective.  No vacuum pumps, no multi-part moulds, nothing very complex.  Push moulds and simple two part moulds for tiny items are the limit of my casting desires at the moment.
So I got some “oyumaru” (also known as “Instant Mold”) and used it as outlined below.  I used nothing but basic techniques and I wouldnt really describe it as a “how-to” or anything, but it might be of interest to someone.

Firstly I selected the replacement part that I wanted to cast: its the hooded and masked head shown on the bottom right in the picture above.

Next I cleaned up mould lines and all of that, filed the underneath of the neck flat and stuck this flat area to a flat piece of plastic (a tiddlywink).

I attached the part to the tiddlywink as I wanted to make sure that I could get the mould rubber to cover the detail on the bottom of the part.  I found it easiest to stick the part to something flat to so that I could work the oyumaru into it while still maintaining a flat base to the mould.  I hoped that this would help to minimise distortion in the castings later while also making the part easier to handle.

Next I got a small bowl of boiling water and the oyumaru.  The oyumaru is the waxy looking long rectangle on the left.   In its room temperature state it has a similar consistency to a low melt glue stick

The oyumaru was submerged in the hot water for a minute or two at which point it became very pliant.  I quickly covered the part in oyumaru.  I ensured that I worked the material under the part and and against the flat plastic tiddlywink.  I also tried to minimise possible air bubbles in the detailed face area by working the oyumaru gently into those details as carefully as I could.

A few minutes later the oyumaru had cooled down and become rigid again.

I prised the part from the tiddlywink.  Then starting at the bottom where the part had been glued to the plastic I used a scalpel to cut through the side of the mould.

I went about two thirds of the way around the mould, gently cutting to the part as I went.  I didnt want to split the mould entirely in two as I didnt want the hassle of having to engineer some way of keying the two halves back together when casting.  Keeping the mould attached would help to minimise that.

Also of note is that I slit the mould from ear to ear across the top of the head, from the parts perspective.  As the head is hooded I figured that those areas would be the easiest to clean flashing and mould lines from later.  Obviously, splitting the mould through highly detailed or recessed areas would make cleaning up the subsequent castings far more difficult (as anyone reading who has ever had to clean a crappy mould line of the side of a miniatures head knows).

I then mixed up some two part epoxy putty (I used ProCreate, which is essentially grey coloured “Green Stuff”) and pushed that into the mould.

Like when making the mould itself, I was careful to make sure that as much of the detailed face area was filled as possible.  The part as it came out of the mould is on the left.  Note the flashing from the cut side of the mould.  Although it looks bad in the photo it really is only a fraction of a millimeter thick, so it took only a moment to remove.

Above is a quick snap of the newly cast head in its new home.  As you can see its a pretty huge figure (from Ramshackle Games) which is why I included the Servitor for comparison.  The size of the target figure isnt really releant anyway, the piece cast was a standard 28mm heroic scale head.

Like that head, the Adeptus Mechanicus cog on the big models chest is also made of ProCreate via an Oyumaru push mould.  That also came out pretty well.

My verdict on this stuff is that its brilliant.  It provides a very simple, low outlay and non-toxic way to replicate miniature parts.  I see this as being part of my hobby toolbox for good.